1. Field of the Invention
The present invention is a method of producing crystalline silicon by cooling and solidifying molten silicon gradually in a unidirectional manner.
2. Discussion of the Background
Polycrystalline silicon solar cells are the type of solar cells which are presently produced in the largest quantity. The performance of a polycrystalline silicon solar cell element depends greatly on the quality of the polycrystalline silicon. For this reason, various improvements have been made in methods for producing polycrystalline silicon, particularly in methods to decrease the level of impurities and to improve the crystallinity of the polycrystalline silicon
Methods for producing polycrystalline silicon are roughly divided into two steps: a step in which high-purity silicon is produced from metallic silicon, and a step of solidifying molten high-purity silicon by a unidirectional solidification process. In conventional processes, in order to reduce the level of impurity elements in the silicon, metallic silicon is reacted with hydrochloric acid to obtain gaseous trichlorosilane, followed by rectifying the gaseous trichlorosilane and then precipitating high-purity silicon from the gas while reducing the rectified gas with hydrogen gas, thereby forming high-purity silicon. Alternatively, or in addition, the level of impurities in the silicon may also be reduced by stirring the molten silicon during the unidirectional solidification step.
For example, the following methods are disclosed for stirring molten silicon. JP-A 61-141612 discloses a method of turning a mold containing the molten silicon, JP-A 5-254817 discloses a method of stirring molten silicon by means of a magnetic field and JP-A 10-182135 discloses a method which comprises blowing an inert gas into the molten silicon by inserting a gas-supplying lance into the molten silicon above the solid-liquid phase boundary between the molten and solid silicon.
However, the above-mentioned first method requires expensive equipment, has a high production cost, and requires difficult and costly equipment maintenance. The second method requires expensive equipment. The third method has the problem that the gas-supplying lance tends to melt and deposit impurities into the molten silicon, resulting in less pure silicon.
The properties of solar cells made from silicon having a high oxygen content are poorer than those of solar cells made from silicon having a lower oxygen content. A certain level of oxygen is dissolved or otherwise incorporated into remelted, lump silicon derived from single crystal silicon scrap, which is a convenient main raw material. It is therefore necessary to decrease the oxygen level in the silicon during the process of melting and unidirectional solidification. Furthermore, when the surface of molten silicon is not covered by an inert gas, a carbon oxide gas (CO), silicon oxide gas (SiO) and the like are absorbed into the molten silicon, thereby increasing the level of carbon and oxygen impurities therein, which results in a degradation of the properties of the resulting solar cells. As defined herein, carbon oxide gas includes any gaseous oxide of carbon, for example carbon monoxide or carbon dioxide, or mixtures thereof, and silicon oxide gas includes any oxide of silicon, for example, silicon dioxide.
The present invention is a method for producing high quality polycrystalline silicon at a low cost, which has low impurity levels and high crystallinity. The following aspects of the present invention further describe the present invention:
(1) The first aspect of the present invention is a process for producing crystalline silicon in which molten silicon crystallizes upward from the inner bottom of a mold by means of a positive temperature gradient extending from the inner bottom of the mold upward, in which the temperature of the silicon increases from the bottom of the mold, upward. During the crystallization of the silicon, an inert gas, such as argon (Ar) is blown onto the surface of the molten silicon from a position above the surface of the molten silicon, in order to vibrate the surface of the molten silicon in such a manner that cavities are formed in the surface. Consequently, the vibration continuously forms a fresh surface on the molten silicon which promotes the discharge to the surrounding atmosphere of SiO gas generated in the interior of the molten silicon, which is effective in removing oxygen (O) impurities in the molten silicon. In addition, because a gas-supplying lance is not inserted into the molten silicon, there is less chance of contaminating the molten silicon with impurities derived from the gas-supplying lance.
(2) The second aspect of the present invention is a process for producing crystalline silicon as mentioned in (1), in which the surface of the molten silicon is covered with an inert gas. By covering the surface of the molten silicon with an inert gas such as Ar, contaminating gases such as CO gas and SiO gas from the surrounding atmosphere are not absorbed by the molten silicon.
(3) The third aspect of the present invention is a process for producing crystalline silicon as mentioned in (1) or (2), in which a susceptor is located above the surface of the molten silicon and the inert gas is introduced into the space between the susceptor and molten silicon through an opening in the susceptor. The inert gas present near the surface of the molten silicon is restrained from diffusing into the surrounding atmosphere by the susceptor, so the inert gas can cover the surface of the molten silicon for a long time. Therefore, the CO gas and SiO gas in the surrounding atmosphere are effectively prevented from entering into the molten silicon by a small flow rate of an inert gas.
(4) The fourth aspect of the present invention is a process for producing crystalline silicon as mentioned in any one of (1) to (3), in which the flow rate of the inert gas blown into the mold is reduced as the solid-liquid phase boundary of the silicon in the mold moves upward. This prevents the solid -liquid phase boundary from being disturbed by the blown inert gas.
(5) The fifth aspect of the present invention is a process for producing crystalline silicon as mentioned in any one of (1) to (4), in which the flow rate of the inert gas blown into the mold, the inside radius of the gas-supplying lance which introduces the inert gas onto the surface of the molten silicon, and the distance from a nozzle of the lance to the surface of the molten silicon satisfy the following formula;
3xe2x89xa6f/(rH)xe2x89xa660
where f (in units of 1/min) is the flow rate of the inert gas;
r (in units of cm) is the inside radius of the lance; and
H (in units of cm) is the distance from a port of the lance to the surface of molten silicon.
The flow rate of the blown gas, the inside radius of the gas-supplying lance and the distance from a port of the lance to the surface of the molten silicon are conditions selected so that the surface of the molten silicon is vibrated effectively, promoting the discharge to the surrounding atmosphere of SiO gas generated in the interior of the molten silicon. Therefore, the level of O impurities in the molten silicon can be reduced.
(6) The sixth aspect of the present invention is a process for producing crystalline silicon as mentioned in any one of (1) to (5), in which there are one or more gas nozzles on the lance, and the number of nozzles increases as the surface area of the molten silicon increases.
In the above-mentioned method for producing crystalline silicon, the surface of the molten silicon is vibrated by the lance nozzles, the number of which is selected according to the surface area of the molten silicon, so as to effectively promote the discharge to the surrounding atmosphere of SiO gas generated in the interior of the molten silicon. This causes a reduction in the O impurity level in the molten silicon.